Process for producing para-xylene

ABSTRACT

A process for producing a PX-rich product, the process comprising: (a) providing a PX-depleted stream; (b) isomerizing at least a portion of the PX-depleted stream to produce an isomerized stream having a PX concentration greater than the PX-depleted stream and a benzene concentration of less than 1,000 ppm and a C 9 + hydrocarbons concentration of less than 5,000 ppm; and (c) separating the isomerized stream by selective adsorption.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/594,286(now U.S. Pat. No. 8,273,934), which claims the benefit of U.S.Provisional Application No. 61/122,570, filed Dec. 15, 2008, bothincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a process for producing para-xylene.

BACKGROUND OF THE INVENTION

Ethylbenzene (EB), para-xylene (PX), ortho-xylene (OX) and meta-xylene(MX) are often present together in C₈ aromatic product streams fromchemical plants and oil refineries. Of these C₈ compounds, although EBis an important raw material for the production of styrene, for avariety of reasons most EB feedstocks used in styrene production areproduced by alkylation of benzene with ethylene, rather than by recoveryfrom a C₈ aromatics stream. Of the three xylene isomers, PX has thelargest commercial market and is used primarily for manufacturingterephthalic acid and terephthalate esters for use in the production ofvarious polymers such as poly(ethylene terephthalate), polypropyleneterephthalate), and poly(butene terephthalate). While OX and MX areuseful as solvents and raw materials for making products such asphthalic anhydride and isophthalic acid, market demand for OX and MX andtheir downstream derivatives is much smaller than that for PX.

Given the higher demand for PX as compared with its other isomers, thereis significant commercial interest in maximizing PX production from anygiven source of C₈ aromatic materials. However, there are two majortechnical challenges in achieving this goal of maximizing PX yield.Firstly, the four C₈ aromatic compounds, particularly the three xyleneisomers, are usually present in concentrations dictated by thethermodynamics of production of the C₈ aromatic stream in a particularplant or refinery. As a result, the PX production is limited, at most,to the amount originally present in the C₈ aromatic stream unlessadditional processing steps are used to increase the amount of PX and/orto improve the PX recovery efficiency. Secondly, the C₈ aromatics aredifficult to separate due to their similar chemical structures andphysical properties and identical molecular weights.

A variety of methods are known to increase the concentration of PX in aC₈ aromatics stream. These methods normally involve recycling the streambetween a separation step, in which at least part of the PX is recoveredto produce a PX-depleted stream, and a xylene isomerization step, inwhich the PX content of the PX-depleted stream is returned back towardsequilibrium concentration, typically by contact with a molecular sievecatalyst. However, the commercial utility of these methods depends onthe efficiency, cost effectiveness and rapidity of the separation stepwhich, as discussed above, is complicated by the chemical and physicalsimilarity of the different C₈ isomers.

Fractional distillation is a commonly used method for separatingdifferent components in chemical mixture. However, it is difficult touse conventional fractional distillation technologies to separate EB andthe different xylene isomers because the boiling points of the four C₈aromatics fall within a very narrow 8° C. range, namely from about 136°C. to about 144° C. (see Table 1 below). In particular, the boilingpoints of PX and EB are about 2° C. apart, whereas the boiling points ofPX and MX are only about 1° C. apart. As a result, large equipment,significant energy consumption, and/or substantial recycles would berequired for fractional distillation to provide effective C₈ aromaticseparation.

TABLE I C₈ compound Boiling Point (° C.) Freezing Point (° C.) EB 136−95 PX 138 13 MX 139 −48 OX 144 −25

Fractional crystallization is an alternative method of separatingcomponents of a mixture and takes advantage of the differences betweenthe freezing points and solubilities of the components at differenttemperatures. Due to its relatively higher freezing point, PX can beseparated as a solid from a C₈ aromatic stream by fractionalcrystallization while the other components are recovered in aPX-depleted filtrate. High PX purity, a key property needed forsatisfactory conversion of PX to terephthalic acid and terephthalateesters, can be obtained by this type of fractional crystallization. U.S.Pat. No. 4,120,911 provides a description of this method. Commerciallyavailable fractional crystallization processes and apparatus include thecrystallization isofining process, the continuous countercurrentcrystallization process, direct CO₂ crystallizer, and scraped drumcrystallizers. Due to high utility usage and the formation of a eutecticbetween PX and MX, it is usually more advantageous to use a feed with ashigh an initial PX concentration as possible when using fractionalcrystallization to recover PX.

An alternative xylene separation method uses molecular sieves, such aszeolites, to selectively adsorbed para-xylene from the C₈ aromaticfeedstream to form a PX-depleted effluent. The adsorbed PX can then bedesorbed by various ways such as heating, lowering the PX partialpressure or stripping. (See generally U.S. Pat. Nos. 3,706,812,3,732,325 and 4,886,929). Two commercially available processes used inmany chemical plants or refineries are PAREX™ and ELUXYL™ processes.Both processes use molecular sieves to adsorb PX. In suchmolecular-sieve based adsorption processes, a higher amount of PX,typically over 90%, compared with that from a fractional crystallizationprocess, typically below 65%, may be recovered from the PX present in aparticular feed.

For many of these PX separation processes, the higher the original PXconcentration in the feed stream, the easier, more efficient and moreeconomical it becomes to perform the PX separation. Therefore, there arestrong economic and technical incentives to increase the PXconcentration in a hydrocarbon feed stream comprising the C₈ aromaticcompounds prior to sending the feed stream to a PX recovery unit.

Known technologies integrate selective adsorption and fractionalcrystallization in PX separation and isomerization loops. For example,U.S. Publication No. 2007/0249882 teaches a process whereby a C₈-richstream is fed to both selective adsorption and fractionalcrystallization recovery units. By taking advantage of the higherrecovery rates of the selective adsorption unit, the PX-depletedeffluent (which is typically below 65% depleted) from the fractionalcrystallization recovery unit can be fed to the selective adsorptionunit. While this yields greater efficiency, further advancements wereachieved by increasing the C₈ concentration of the fractionalcrystallization PX-depleted effluent stream prior to further recovery byselective adsorption. This was achieved by a xylene isomerization stepbetween the fractional crystallization and the selective adsorptionunits. Furthermore, liquid-phase isomerization was preferred in thisstep because of cost, simplicity (no need for hydrogen recycle), and lowxylene loss.

The liquid-phase isomerization of C₈ aromatics to increase the PXconcentration is temperature dependent. That is, the conversionefficiency to PX increases with increasing temperature with the primarycontrolling limitation being the ability to maintain a liquid state.Although increased system pressure can allow for higher temperatures,physical and cost constraints ultimately place limitations on theprocess. As a result, liquid-phase isomerization typically operates near300° C. and at pressures above 300 psig.

While integrated systems of this type yield advantageous efficiencies,the selective adsorption unit is intolerant to C₇− hydrocarbons,particularly benzene, and C₉+ aromatic compounds (9 or more carbonsaromatic compounds). In fact, most selective adsorption unitscommercially employed can only tolerate up to about 300 ppm of benzeneand less than 5000 ppm C₉+ aromatic compounds. Thus, the production ofC₇− hydrocarbons or C₉+ aromatics by liquid-phase isomerizationprocesses, particularly those positioned between fractionalcrystallization and selective adsorption, presents additional problemsfor overall process efficiency.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a process for producing aPX-rich product, the process comprising: (a) separating a feedstockcontaining C₈ hydrocarbons to produce a C₈ hydrocarbons rich stream; (b)separating a first portion of the C₈ hydrocarbons rich stream to producea first PX-rich stream and a first raffinate stream; (c) isomerizing atleast a portion of the first raffinate stream to produce a firstisomerized stream having a higher PX concentration than the firstraffinate stream; (d) separating a second portion of the C₈ hydrocarbonsrich stream to produce a second PX-rich stream and a second raffinatestream; (e) isomerizing, at least partially in the liquid phase, atleast a portion of the second raffinate stream to produce a secondisomerized stream having a higher PX concentration than the secondraffinate stream, where the second isomerized stream has a benzeneconcentration of less than 1,000 ppm and a C₉+ hydrocarbonsconcentration of less than 5,000 ppm; (f) recovering at least a portionof at least one of the first and second PX-rich streams as PX-richproduct; and (g) providing at least a portion of the first isomerizedstream and at least a portion of the second isomerized stream to theseparating (a).

Other embodiments provide a process for producing a PX-rich product, theprocess comprising: (a) separating a feedstock containing C₈hydrocarbons to produce a C₈ hydrocarbons rich stream; (b) separating afirst portion of the C₈ hydrocarbons rich stream to produce a firstPX-rich stream and a first raffinate stream; (c) isomerizing at least aportion of the first raffinate stream to produce a first isomerizedstream having a higher PX concentration than the first raffinate stream;(d) separating the first isomerized stream to produce a second C₈hydrocarbons rich stream; (e) separating the second C₈ hydrocarbons richstream to produce a second PX-rich stream and a second raffinate stream;(f) isomerizing, at least partially in the liquid phase, at least aportion of the second raffinate stream to produce a second isomerizedstream having a higher PX concentration than the second raffinatestream; and (g) providing at least a portion of the second isomerizedstream to the separating (a).

Other embodiments provide a process for producing a PX-rich product, theprocess comprising: (a) separating a feedstock containing C₈hydrocarbons to produce a C₈ hydrocarbons rich stream; (b) separating afirst portion of the C₈ hydrocarbons rich stream to produce a firstPX-rich stream and a first raffinate stream; (c) isomerizing at least aportion of the first raffinate stream to produce a first isomerizedstream having a higher PX concentration than the first raffinate stream;(d) separating the first raffinate stream to produce a second C₈hydrocarbons rich stream; (e) separating the second C₈ hydrocarbons richstream to produce a second PX-rich stream and a third stream; (f)isomerizing, at least partially in the liquid phase, at least a portionof the second raffinate stream to produce a second isomerized streamhaving a higher PX concentration than the second raffinate stream, wherethe second isomerized stream has a benzene concentration of less than1,000 ppm and a C₉+ hydrocarbons concentration of less than 5,000 ppm;and (g) providing at least a portion of the second isomerized stream tothe separating (a).

Other embodiments provide a PX-production process where a C₈ hydrocarbonrich stream is separated into a PX-rich stream and a PX-depleted streamby fractional crystallization, and where the PX-depleted stream issubsequently isomerized to increase the concentration of PX in thePX-depleted stream for subsequent separation by selective adsorption,the improvement comprising isomerizing said PX-depleted stream in theliquid phase to produce a stream having a higher PX concentration thanthe PX-depleted stream, a benzene concentration of less than 500 ppm,and a C₉+ concentration of less than 5,000 ppm.

Other embodiments provide a process for producing a PX-rich product, theprocess comprising: (a) providing a PX-depleted stream; (b) isomerizingat least a portion of the PX-depleted stream to produce an isomerizedstream having a PX concentration greater than the PX-depleted stream anda benzene concentration of less than 1,000 ppm and a C₉+ hydrocarbonsconcentration of less than 5,000 ppm; and (c) separating the isomerizedstream by selective adsorption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a producing para-xylene by employingintegrated fractional crystallization and selective adsorption accordingto one or more embodiments of the present invention.

FIG. 2 is a schematic diagram of a producing para-xylene by employingintegrated fractional crystallization and selective adsorption accordingto one or more embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

All patents, patent applications, test procedures, priority documents,articles, publications, manuals, and other documents cited herein arefully incorporated by reference to the extent such disclosure is notinconsistent with the present application and for all jurisdictions inwhich such incorporation is permitted.

When numerical lower limits and numerical upper limits are listedherein, ranges from any lower limit to any upper limit are contemplated.

One having ordinary skill in the art understands that the embodimentsdiscussed in this application do not represent all the possibleapparatus or process variations embodied by the present disclosure. Inaddition, many pieces of equipment and apparatus and certain processingsteps may be needed for industrial, commercial or even experimentalpurposes. Examples of such equipments and apparatus and processing stepsare, but not limited to, distillation columns, fractionation columns,heat exchanges, pumps, valves, pressure gauges, temperature gauges,liquid-vapor separators, feed and product driers and/or treaters, claytreaters, feed and/or product storage facilities, and processes andsteps for process control. While such equipment, apparatus and stepsthat are not needed for understanding the essence of the presentapplication are not shown in the drawings, some of them may be mentionedfrom time to time to illustrate various aspects of the disclosure. It isalso noted that some of the equipment may be placed at different placesin the process depending on the conditions of the processes.

As used herein, the term “C₈+ hydrocarbons” means hydrocarbons havingeight or more carbon atoms per molecule. A C₈+ hydrocarbons feed and/orproduct is a hydrocarbon feed and/or product having more than 10 wt. %,such as more than 20 wt. %, for example more than 40 wt. %, such as morethan 50 wt. %, and in some cases more than 80 wt. %, C₈+ hydrocarbons inthe feed and/or product. The term “C₉+ hydrocarbons” as used hereinmeans hydrocarbons having nine or more carbon atoms per molecule andincludes C₉+ aromatics, which are aromatic compounds including 9 or morecarbon atoms per molecule. A C₉+ hydrocarbons feed and/or product is ahydrocarbon feed and/or product having more than 10 wt. %, such as morethan 20 wt. %, for example more than 40 wt. %, such as more than 50 wt.%, and in some cases more than 80 wt. %, C₉+ hydrocarbons in the feedand/or product. The term “C₇− hydrocarbons” as used herein meanshydrocarbons having seven or less carbon atoms per molecule. A C₇−hydrocarbons feed and/or product is a hydrocarbon feed and/or producthaving more than 10 wt. %, such as more than 20 wt. %, for example morethan 40 wt. %, such as more than 50 wt. %, and in some cases more than80 wt. %, C₇− hydrocarbons in the feed and/or product. The term “C₈hydrocarbons” as used herein means hydrocarbons having eight carbonatoms per molecule, including PX. A C₈ hydrocarbons feed and/or product,with the exception of a PX-rich or PX-depleted stream and/or product, isa hydrocarbon feed and/or product having more than 10 wt. %, such asmore than 20 wt. %, for example more than 40 wt. %, such as more than 50wt. %, and in some cases more than 80 wt. %, C₈ hydrocarbons in the feedand/or product. The term “C₈ aromatic hydrocarbons” as used herein meansaromatic hydrocarbons having eight carbon atoms per molecule, i.e.,xylene(s) and/or EB. A C₈ aromatic hydrocarbons feed and/or product,with the exception of a PX-rich or PX-depleted stream and/or product, isa hydrocarbon feed and/or product having more than 10 wt. %, such asmore than 20 wt. %, for example more than 40 wt. %, such as more than 50wt. %, and in some cases more than 80 wt. %, C₈ aromatic hydrocarbons inthe feed and/or product.

The term “PX-depleted” means that PX concentration in an exiting streamof a particular unit is lowered as compared to the concentration in afeed stream to the same unit. It does not mean that all of PX has to bedepleted or removed from the xylenes-containing feed stream(s) to theunit. The term “PX-rich” means that PX concentration in an exitingstream of a particular unit is increased as compared to theconcentration in a feed stream to the same unit. It does not mean thatthe PX concentration has to be 100%.

Feedstock

The feedstock employed in the present process may be any C₈+ hydrocarbonfeedstock containing C₈ aromatic hydrocarbons, such as a reformatestream, a hydrocracking product stream, a xylene or EB reaction productstream, an aromatic alkylation product stream, an aromaticdisproportionation stream, an aromatic transalkylation stream, and/or aCyclar™ process stream. The feedstock may further comprise recyclestream(s) from the isomerization step(s) and/or various separatingsteps. The C₈+ hydrocarbon feedstock comprises PX, together with MX, OX,and/or EB. In addition to xylenes and EB, the C₈+ hydrocarbon feedstockmay also contain certain amounts of other aromatic or even non-aromaticcompounds. Examples of such aromatic compounds are benzene, toluene andC₉+ aromatics such as mesitylene, pseudo-cumene and others. These typesof feedstream(s) are described in “Handbook of Petroleum RefiningProcesses”, Eds. Robert A. Meyers, McGraw-Hill Book Company, SecondEdition, all relevant parts of which are hereby incorporated byreference.

Process Description

In one or more embodiments, the process of the present applicationcomprises an initial separating step that serves to remove the C₉+hydrocarbons from the C₈+ hydrocarbon feedstock. Because of thedifferences in molecular weights, boiling points and other physical andchemical properties, the C₉+ hydrocarbons compounds, aromatic ornon-aromatic, can be separated relatively easily from the xylenes andEB. Generally, therefore, the first separating step includes fractionaldistillation, although other separation methods, such ascrystallization, adsorption, a reactive separation, a membraneseparation, extraction, or any combination thereof, can also be used.These separation methods are described in “Perry's Chemical Engineers'Handbook”, Eds. R. H. Perry, D. W. Green and J. O. Maloney, McGraw-HillBook Company, Sixth Edition, 1984, and “Handbook of Petroleum RefiningProcesses”, Eds. Robert A. Meyers, McGraw-Hill Book Company, SecondEdition, all relevant parts of which are hereby incorporated byreference.

In one or more embodiments, after removal of the C₉+ hydrocarbons, theprocess comprises at least one separating step to recover a PX-richproduct stream from the resultant C₈ hydrocarbon stream. In oneembodiment, the PX-rich product stream comprises at least 50 wt. % PX,preferably at least 60 wt. % PX, more preferably at least 70 wt. % PX,even preferably at least 80 wt. % PX, still even preferably at least 90wt. % PX, and most preferably at least 95 wt. % PX, based on the totalweight of the PX-rich product stream.

In one or more embodiments, the separating step to recover the PX-richproduct stream is performed in a PX recovery unit comprising at leastone a crystallization unit, an adsorption unit such as a PAREX™ unit oran ELUXYL™ unit, a reactive separation unit, a membrane separation unit,an extraction unit, a distillation unit, a fractionation unit, or anycombination thereof. These types of separation unit(s) and their designsare described in “Perry's Chemical Engineers' Handbook”, Eds. R. H.Perry, D. W. Green and J. O. Maloney, McGraw-Hill Book Company, SixthEdition, 1984, and “Handbook of Petroleum Refining Processes”, Eds.Robert A. Meyers, McGraw-Hill Book Company, Second Edition, all relevantparts of which are hereby incorporated by reference.

In one or more embodiments, further separating steps employed in thepresent process serve to separate a C₈ hydrocarbon feedstream into aPX-rich effluent stream and a PX-depleted stream. These separating stepsmay be performed in separating units comprising at least one of acrystallization unit, an adsorption unit such as a PAREX™ unit or anELUXYL™ unit, a reactive separation unit, a membrane separation unit, anextraction unit, a distillation unit, a fractionation unit, or anycombination thereof. These types of separation unit(s) and their designsare described in “Perry's Chemical Engineers' Handbook”, Eds. R. H.Perry, D. W. Green and J. O. Maloney, McGraw-Hill Book Company, SixthEdition, 1984, and “Handbook of Petroleum Refining Processes”, Eds.Robert A. Meyers, McGraw-Hill Book Company, Second Edition, all relevantparts of which are hereby incorporated by reference.

In one or more embodiments, the process of the present applicationincludes at least two isomerization steps, in each of which a feedstream comprising C₈ aromatic compounds is isomerized to produce anisomerization effluent. Typically, the feed stream to each isomerizationstep comprises PX in a concentration below its equilibrium concentrationrelative to other inter-convertible C₈ aromatic compounds under theisomerization conditions. Each catalyzed isomerization step serves toincrease the PX concentration to near its equilibrium level. Theisomerization step may also serve to convert part or all of EB presentin the feed stream to benzene and light hydrocarbons (i.e., hydrocarbonshaving less than 6 carbons per molecule). Alternatively, theisomerization step may also serve to isomerize part or all of EB presentin the feed stream to xylene(s).

There are many catalysts or combinations of catalysts that can be usedin each isomerization step to effect the desired reaction. There aregenerally two types of xylene isomerization catalysts. One type ofisomerization catalyst can more or less equilibrate the four differentC₈ aromatic compounds, including EB, to the concentrations dictated bythermodynamics under the reaction conditions. This allows maximumformation of PX from C₈ aromatics in a particular feed. Examples ofthese type catalysts include IFP/Engelhard Octafining™ and OctafiningII™ catalysts used in the respective processes. The other type of xyleneisomerization catalyst can effect EB conversion in addition to xyleneisomerization, generally in the presence of hydrogen. As discussedearlier, this type of catalyst will remove EB and produce benzene andethane as byproducts. This may be a desirable disposition of EB,depending on supplies and demands of various products as well as otherequipment present in a particular plant. Examples include Mobil HighTemperature Isomerization (MHTI™) catalysts, Mobil High ActivityIsomerization catalysts (MHAI™) and UOP ISOMAR™ I-100 catalysts.

A number of suitable isomerization reactors may be used for the presentdisclosure. Some non-limiting examples are described in U.S. Pat. Nos.4,899,011 and 4,236,996.

For the present disclosure, a xylene isomerization reaction may becarried out in a liquid phase, a vapor (gas) phase, a super criticalphase, or a combination thereof. The selection of isomerization reactionconditions and the specific composition of the aromatic feed streambeing isomerized determine the physical state of the aromatic feedstream in the xylene isomerization reactor.

In one or more embodiments, the present invention provides aPX-production and recovery system that includes an integrated selectiveadsorption and fractional crystallization loop with a liquid-phaseisomerization process that treats the PX-depleted effluent from thefractional crystallization process. In one or more embodiments, theliquid-phase isomerization process is manipulated to reduce theproduction of C₇− hydrocarbon byproducts, particularly benzene, and C₉+aromatic byproducts from the liquid-phase isomerization process.Practice of the present invention yields an overall system that isunexpectedly more efficient than prior art systems.

An exemplary process according to one embodiment of the presentinvention can be described with reference to FIG. 1, which includes afirst separating unit 301, which may be one or more distillationcolumns. Separating unit receives a C₈+ aromatic hydrocarbon feed streamfrom line 302 and separates the feed into an overhead vapor stream vialine 304 and a bottom liquid stream via line 303. The bottom liquidstream is composed mainly of C₉+ hydrocarbons (e.g., C₉+ aromatics) andsome ortho-xylene (OX), and it is removed from the first separating unit301 through line 303 for further processing such as distillations for OXrecovery. The overhead stream is composed mainly of C₈ aromatichydrocarbons (typically about 50% meta xylene (MX), about 20% PX, about15% OX and about 15% EB) and is removed from the first separating unit301 through line 304 and is sent for PX recovery.

PX recovery in the process shown in FIG. 1 is effected by both afractional crystallization unit 308 and a selective adsorption unit 309.Thus at least one part of the C₈ aromatic hydrocarbon removed from thefirst separating unit 301 through line 304 is fed by line 306 to thefractional crystallization unit 308, where a first PX-rich productstream is recovered through line 310 and a PX-depleted raffinate streamis withdrawn via line 311.

The remainder of the C₈ aromatic hydrocarbon removed from the firstseparating unit 301 is fed to the selective adsorption unit 309 throughline 304. In one or more embodiments, the C₈ aromatic hydrocarbonremoved from first separating unit 301 is combined with the PX-depletedraffinate stream from fractional crystallization unit 308, which may becombined at line 304 via line 312. From selective adsorption unit 309, asecond PX-rich stream is recovered through line 313 and a furtherPX-depleted (raffinate) stream is withdrawn via line 314. The furtherPX-depleted stream is fed by line 314 to a xylene isomerization unit 315where the stream is converted into an isomerized stream having higher PXconcentration than that of the further stream.

The isomerized stream is removed from the xylene isomerization unit 315by line 316 and is fed to separation unit 301 optionally through line302. In one or more embodiments, one or more separating units 320 mayoptionally be provided between the xylene isomerization unit 315 andfirst separating unit 301 to remove constituents such as C₇−hydrocarbons or C₉+ hydrocarbons that may be generated by the xyleneisomerization unit 315.

A second xylene isomerization unit 317 treats all or part of thePX-depleted raffinate stream withdrawn from the fractionalcrystallization unit 308 via line 311. When unit 317 treats part of thePX-depleted raffinate stream, the remaining portion of the raffinatestream could be sent to the selective adsorption unit 309 via line 312and optionally 304.

In one or more embodiments, second xylene isomerization unit 317 is aliquid-phase isomerization unit. The effluent stream from the secondxylene isomerization unit 317, which has a higher PX concentration thanthe PX-depleted raffinate stream, is withdrawn through line 318 andultimately fed to selective adsorption unit 309. In this way, theoverall PX content of the feed to selective adsorption unit 309 can beincreased. In one or more embodiments, the liquid-phase isomerizationproduct contains mostly equilibrium or near equilibrium xylene.

In one or more embodiments, all or part of the effluent stream fromsecond xylene isomerization unit 317 is sent directly to firstseparating unit 301 via 318. In one or more embodiments, line 318 maydirect the effluent to a tray position to affect the separation of C₈and C₉+ hydrocarbons.

In other embodiments, all or part of the effluent stream from secondxylene isomerization unit 317 is sent directly to selective adsorptionunit 309 via line 319 and optionally line 304. In one or moreembodiments, the effluent stream from second xylene isomerization unit317 (i.e., the isomerized stream) is sent directly to selectiveadsorption unit 309 without further processing (particularly processingto remove or reduce C₇− or C₉+ hydrocarbons). In other embodiments, theisomerized stream from isomerization unit 317 is further treated, suchas by fractionation, to remove at least a part of the C₇− and C₉+hydrocarbons formed in isomerization unit 317 and then sent directly toselective adsorption unit 309.

In one or more embodiments, the operation of liquid-phase isomerizationunit 317 is manipulated to produce a PX-rich stream having less than1,000 ppm, in other embodiments less than 500 ppm, and in otherembodiments less than 300 ppm, and in other embodiments less than 250ppm benzene.

In one or more embodiments, the operation of liquid-phase isomerizationunit 317 is manipulated to produce a PX-rich stream having less than5,000 ppm in other embodiments less than 4,000 ppm, in other embodimentsless than 3,000 ppm, in other embodiments less than 2,000 ppm, in otherembodiments less than 1,000 ppm, and in other embodiments less than 500ppm C₉+ hydrocarbons (e.g., C₉+ aromatics).

In one or more embodiments, liquid-phase isomerization unit 317 isoperated at or below 300° C., in other embodiments below 280° C., and inother embodiments below 260° C. In these or other embodiments,liquid-phase isomerization unit 317 is operated at or above 150° C., inother embodiments above 175° C., and in other embodiments above 200° C.

In one or more embodiments, liquid-phase isomerization unit 317 isoperated at a pressure below 350 psi, in other embodiments below 300psi, and in other embodiments below 270 psi. In these or otherembodiments, liquid-phase isomerization unit 317 is operated at apressure above 30 psi, in other embodiments above 75 psi, and in otherembodiments above 125 psi.

In one or more embodiments, the flow rate of the PX-depleted raffinatestream through isomerization unit 317 is at least 0.1 weight hourlyspace velocity (WHSV), in other embodiments at least 1.0 WHSV, in otherembodiments at least 2.0 WHSV, and in other embodiments at least 5.0WHSV. In these or other embodiments, the flow rate of the PX-depletedraffinate stream through isomerization unit 317 is less than 100 WHSV,and in other embodiments less than 50 WHSV, in other embodiments lessthan 10 WHSV.

In one or more embodiments, liquid-phase isomerization unit 317 includesa solid acid catalyst. Solid acid is described by the Br{acute over(ø)}nsted and Lewis definitions as any material capable of donating aproton or accepting an electron pair. This description can be found inK. Tanabe. Solid Acids and Bases: their catalytic properties. Tokyo:Kodansha Scientific, 1970, p. 1-2. This reference is incorporated hereinby reference in its entirety. The solid acid suitable for this inventioncan comprise at least one of solid acid, supported acid, or mixturesthereof. The solid acid can comprise nonporous, microporous, mesoporous,macroporous or as a mixture thereof. These porosity designations areIUPAC conventions and are defined in K. S. W. Sing, D. H. Everett, R. A.W. Haul L. Moscou, R. A. Pierotti, J. Rouquérol, T. Siemieniewska,Pure&Appl. Chem. 1995, 57(4), pp. 603-619, which is incorporated hereinby reference in its entirety.

Non-limiting examples of solid acid components are natural clays such askaolinite, bentonite, attapulgite, montmorillonite, clarit, fuller'searth, cation exchange resins and SiO₂.Al₂O₃, B₂O₃.Al₂O₃, Cr₂O₃.Al₂O₃,MoO₃.Al₂O₃, ZrO₂.SiO₂, Ga₂O₃.SiO₂, BeO.SiO₂, MgO.SiO₂, CaO.SiO₂,SrO.SiO₂, Y₂O₃.SiO₂, La₂O₃.SiO₂, SnO.SiO₂, PbO.SiO₂, MoO₃.Fe₂ (MoO₄)₃,MgO.B₂O₃, TiO₂.ZnO, ZnO, Al₂O₃, TiO₂, CeO₂, As₂O₃, V₂O₅, SiO₂, Cr₂O₃,MoO₃, ZnS, CaS, CaSO₄, MnSO₄, NiSO₄, CuSO₄, CoSO₄, CdSO₄, SrSO₄, ZnSO₄,MgSO₄, FeSO₄, BaSO₄, KHSO₄, K₂SO₄, (NH₄)₂SO₄, Al₂(SO₄)₃, Fe₂(SO₄)₃,Cr₂(SO₄)₃, Ca(NO₃)₂, Bi(NO₃), Zn(NO₃)₂, Fe(NO₃)₃, CaCO₃, BPO₄, FePO₄,CrPO₄, Ti₃(PO₄)₄, Zr₃(PO₄)₄, Cu₃(PO₄)₂, Ni₃(PO₄)₂, AlPO₄, Zn₃(PO₄)₂,Mg₃(PO₄)₂, AlCl₃, TiCl₃, CaCl₂, AgCl₂, CuCl, SnCl₂, CaF₂, BaF₂, AgClO₄,and Mg(ClO₄)₂. Depending on the synthesis conditions, these materialscan be prepared as nonporous, microporous, mesoporous, or macroporous,as defined in the reference cited above. Conditions necessary to thesepreparations are known to those of ordinary skill in the art.

Non-limiting examples of solid acids can also include both natural andsynthetic molecular sieves. Molecular sieves have silicate-basedstructures (“zeolites”) and AlPO-based structures. Some zeolites aresilicate-based materials which are comprised of a silica lattice and,optionally, alumina combined with exchangeable cations such as alkali oralkaline earth metal ions. For example, faujasites, mordenites andpentasils are non-limiting illustrative examples of such silicate-basedzeolites. Silicate-based zeolites are made of alternating SiO₂ andMO_(x) tetrahedral, where in the formula M is an element selected fromGroups 1 through 16 of the Periodic Table (new IUPAC). These types ofzeolites have 8-, 10- or 12-membered ring zeolites, such as ZSM-5,ZSM-22, ZSM-48 and ZSM-57.

Other silicate-based materials suitable for use in practicing thepresent invention include zeolite bound zeolites as described in WO97/45387, incorporated herein by reference in its entirety. Thesematerials comprise first crystals of an acidic intermediate pore sizefirst zeolite and a binder comprising second crystals of a secondzeolite. Unlike zeolites bound with amorphous material such as silica oralumina to enhance the mechanical strength of the zeolite, the zeolitebound zeolite catalyst does not contain significant amounts ofnon-zeolitic binders.

The first zeolite used in the zeolite bound zeolite catalyst is anintermediate pore size zeolite. Intermediate pore size zeolites have apore size of from about 5 to about 7 Å and include, for example, AEL,MFI, MEL, MFS, MEI, MTW, EUO, MTT, HEU, FER, and TON structure typezeolites. These zeolites are described in Atlas of Zeolite StructureTypes, eds. W. H. Meier and D. H. Olson, Butterworth-Heineman, ThirdEdition, 1992, which is incorporated herein by reference. Non-limiting,illustrative examples of specific intermediate pore size zeolites areZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-34, ZSM-35, ZSM-38, ZSM-48,ZSM-50 AND ZSM-57. Preferred first zeolites are galliumsilicate zeoliteshaving an MFI structure and alumiminosilicate zeolites having an MFRstructure.

The second zeolite used in the zeolite bound zeolite structure willusually have an intermediate pore size (e.g., about 5.0 to about 5.5 Å)and have less activity than the first zeolite. Preferably, the secondzeolite will be substantially non-acidic and will have the samestructure type as the first zeolite. The preferred second zeolites arealuminosilicate zeolites having a silica to alumina mole ratio greaterthan 100 such as low acidity ZSM-5. If the second zeolite is analuminosilicate zeolite, the second zeolite will generally have a silicato alumina mole ratio greater than 100:1, e.g., 500:1; 1,000:1, etc.,and in some applications will contain no more than trace amounts ofalumina. The second zeolite can also be silicalite, i.e., a MFI typesubstantially free of alumina, or silicalite 2, a MEL type substantiallyfree of alumina. The second zeolite is usually present in the zeolitebound zeolite catalyst in an amount in the range of from about 10% to60% by weight based on the weight of the first zeolite and, morepreferably, from about 20% to about 50% by weight. The second zeolitecrystals, in addition to binding the first zeolite particles andmaximizing the performance of the catalyst may, for example, intergrowand form an overgrowth that coats or partially coats the first zeolitecrystals. In particular embodiments, the crystals will be resistant toattrition.

The zeolite bound zeolite catalyst may be prepared by a three stepprocedure. The first step involves the synthesis of the first zeolitecrystals prior to converting it to the zeolite bound zeolite catalyst.Next, a silica-bound aluminosilicate zeolite can be prepared, forexample, by mixing a mixture comprising the aluminosilicate crystals, asilica gel or sol, water and optionally an extrusion aid and,optionally, a metal component until a homogeneous composition in theform of an extrudable paste develops. The final step is the conversionof the silica present in the silica-bound catalyst to a second zeolitewhich serves to bind the first zeolite crystals together.

It is to be understood that the above description of zeolite boundzeolites can be equally applied to non-zeolitic molecular sieves (i.e.,AlPO's).

Other molecular sieve materials suitable for this invention includealuminophosphate-based materials. Aluminophosphate-based materials aremade of alternating AlO4 and PO4 tetrahedra. Members of this family have8—(e.g., AlPO₄-12, -17, -21, -25, -34, -42, etc.) 10—(e.g., AlPO₄-11,41, etc.), or 12—(AlPO₄-5, -31 etc.) membered oxygen ring channels.Although AlPO₄s are neutral, substitution of Al and/or P by cations withlower charge introduces a negative charge in the framework, which iscountered by cations imparting acidity.

By turn, substitution of silicon for P and/or a P—Al pair turns theneutral binary composition (i.e., Al, P) into a series ofacidic-ternary-composition (Si, Al, P) based SAPO materials, such asSAPO-5, -11, -14, -17, -18, -20, -31, -34, -41, -46, etc. Acidic ternarycompositions can also be created by substituting divalent metal ions foraluminum, generating the MeAPO materials. Me is a metal ion which can beselected from the group consisting of, but not limited to Mg, Co, Fe, Znand the like. Acidic materials such as MgAPO (magnesium substituted),CoAPO (cobalt substituted), FeAPO (iron substituted), MnAPO (manganesesubstituted) ZnAPO (zinc substituted) etc. belong to this category.Substitution can also create acidic quaternary-composition basedmaterials such as the MeAPSO series, including FeAPSO (Fe, Al, P, andSi), MgAPSO (Mg, Al, P, Si), MnAPSO, CoAPSO, ZnAPSO (Zn, Al, P, Si),etc. Other substituted aluminophosphate-based materials include ElAPOand ElAPSO (where El=B, As, Be, Ga, Ge, Li, Ti, etc.). As mentionedabove, these materials have the appropriate acidic strength forreactions such as cracking. The more preferred aluminophosphate-basedmaterials include 10- and 12-membered ring materials (SAPO-11, -31, -41;MeAPO-11, -31, -41; MeAPSO-11, -31, 41; ElAPO-11, -31, -41; ElAPSO-11,-31, -41, etc.) which have significant olefin selectivity due to theirchannel structure.

Supported acid materials are either crystalline or amorphous materials,which may or may not be themselves acidic, modified to increase the acidsites on the surface. Non-limiting, illustrative examples are H₂SO₄,H₃PO₄, H₃BO₃, CH₂(COOH)₂, mounted on silica, quartz, sand, alumina ordiatomaceous earth, as well as heteropoly acids mounted on silica,quartz, sand, alumina or diatomaceous earth. Non-limiting, illustrativeexamples of crystalline supported acid materials are acid-treatedmolecular sieves, sulfated zirconia, tungstated zirconia, phosphatedzirconia and phosphated niobia.

Although the term “zeolites” includes materials containing silica andoptionally, alumina, it is recognized that the silica and aluminaportions may be replaced in whole or in part with other oxides. Forexample, germanium oxide, tin oxide, phosphorus oxide, and mixturesthereof can replace the silica portion. Boron, oxide, iron oxide,gallium oxide, indium oxide, and mixtures thereof can replace thealumina portion. Accordingly, “zeolite” as used herein, means not onlymaterials containing silicon and, optionally, aluminum atoms in thecrystalline lattice structure thereof, but also materials which containsuitable replacement atoms for such silicon and aluminum, such asgallosilicates, borosilicates, ferrosilicates, and the like.

Besides encompassing the materials discussed above, “zeolites” alsoencompasses aluminophosphate-based materials.

Mesoporous solid acids can be ordered and/or non-ordered. Non-limitingexamples of ordered mesoporous materials include pillared layered clays(PILC's), MCM-41 and MCM-48. Non-limiting examples of non-orderedmesoporous materials include silica and titania-based xerogels andaerogels.

The solid acid can also include ordered mesoporous amorphous materials.Non-limiting examples of ordered mesoporous materials include pillaredlayered clays (PILC's), MCM-41 and MCM-48.

An alternate embodiment is shown in FIG. 2, which process includes afirst separating unit 401, which may be one or more distillationcolumns, that receives a C₈+ aromatic hydrocarbon feed stream from line402 and separates the feed into an overhead vapor stream via line 404and a bottom liquid stream via 403 in similar fashion to that processdescribed in FIG. 1.

The overhead vapor stream, which is composed mainly of C₈ aromatichydrocarbons, is fed via line 404 to the selective adsorption unit 409,where a PX-rich product stream is recovered through line 413 and aPX-depleted stream is withdrawn via line 414. The PX-depleted stream isfed by line 414 to a xylene isomerization unit 415 where the stream isconverted into an isomerized stream having higher PX concentration thanthat of the stream fed to isomerization unit 415.

The isomerized stream is removed from xylene isomerization unit 415 byline 416 and is fed to a separating unit 420, which may be one or moredistillation columns. Separation unit 420 separates and transfers C₇−hydrocarbons via line 422, C₉+ hydrocarbons via line 423, and C₈hydrocarbons via line 421. The C₈ hydrocarbon rich stream is fed by line421 to a fractional crystallization unit 408, where a PX-rich productstream is recovered through line 410 and a PX-depleted raffinate streamis withdrawn via line 411.

A second xylene isomerization unit 417 treats all or part of thePX-depleted raffinate stream withdrawn from the fractionalcrystallization unit 408 via line 411. When unit 417 treats part of thePX-depleted raffinate stream, the remaining portion of the raffinatestream could be sent to the selective adsorption unit 409 via 412 andoptionally 404.

In one or more embodiments, second xylene isomerization unit 417 is aliquid-phase isomerization unit. The effluent stream from second xyleneisomerization unit 417, which has a higher PX concentration than thePX-depleted raffinate stream, is withdrawn through line 418 andultimately fed to selective adsorption unit 409. In this way, theoverall PX content of the feed recycled back to selective adsorptionunit 409 can be increased. In one or more embodiments, the liquid-phaseisomerization product contains mostly equilibrium or near equilibriumxylenes.

In one or more embodiments, all or part of the effluent stream fromsecond xylene isomerization unit 417 is sent directly to firstseparating unit 401 via 418. In one or more embodiments, line 418 maydirect the effluent to a proper tray position to affect the separationof C₈ and C₉+ hydrocarbons.

In other embodiments, all or part of the effluent stream from secondxylene isomerization unit 417 is sent directly to selective adsorptionunit 409 via 419 and optionally line 404. In one or more embodiments,the effluent stream from second xylene isomerization unit 417 (i.e., theisomerized stream) is sent directly to selective adsorption unit 409without further processing (particularly processing to remove or reduceC₇− or C₉+ hydrocarbons). In other embodiments, the isomerized streamfrom isomerization unit 417 is further treated, such as byfractionation, to remove at least a part of the C₇− and C₉+ hydrocarbonsformed in isomerization unit 417 and then sent directly to selectiveadsorption unit 409.

Thus, the process depicted in FIG. 2 likewise includes an integratedfractional crystallization unit and selective adsorption unit with aliquid-phase isomerization process increasing the PX concentration ofthe feed effluent stream from the factional crystallization that isdirected toward the selective adsorption unit.

As with the previous embodiments, the isomerization process withinxylene isomerization unit 417 is manipulated to produce a PX-rich streamhaving less than 1,000 ppm, in other embodiments less than 500 ppm, andin other embodiments less than 300 ppm, and in other embodiments lessthan 250 ppm benzene.

In one or more embodiments, the operation of liquid-phase isomerizationunit 417 is manipulated to produce a PX-rich stream having less than5,000 ppm, in other embodiments less than 4,000 ppm, in otherembodiments less than 3,000 ppm, in other embodiments less than 2,000ppm, in other embodiments less than 1,000 ppm, and in other embodimentsless than 500 ppm C₉+ hydrocarbons (e.g., aromatics).

In one or more embodiments, liquid-phase isomerization unit 417 isoperated at or below 300° C., in other embodiments below 280° C., and inother embodiments below 260° C. In these or other embodiments,liquid-phase isomerization unit 417 is operated at or above 150° C., inother embodiments above 175° C., and in other embodiments above 200° C.

In one or more embodiments, liquid-phase isomerization unit 417 isoperated at a pressure below 350 psi, in other embodiments below 300psi, and in other embodiments below 270 psi. In these or otherembodiments, liquid-phase isomerization unit 417 is operated at apressure above 30 psi, in other embodiments above 75 psi, and in otherembodiments above 125 psi.

In one or more embodiments, the flow rate of the PX-depleted raffinatestream through isomerization unit 417 is at least 0.1 weight hourlyspace velocity (WHSV), in other embodiments at least 1.0 WHSV, in otherembodiments at least 2.0 WHSV, and in other embodiments at least 5.0WHSV. In these or other embodiments, the flow rate of the PX-depletedraffinate stream through isomerization unit 417 is less than 100 WHSV,and in other embodiments less than 50 WHSV, in other embodiments lessthan 10 WHSV.

In one or more embodiments, liquid-phase isomerization unit 417 includesa solid acid catalyst as described above with respect to the previousembodiments.

Specific Embodiments

Paragraph A: A process for producing a PX-rich product, the processcomprising: (a) separating a feedstock containing C₈ hydrocarbons toproduce a C₈ hydrocarbons rich stream; (b) separating a first portion ofthe C₈ hydrocarbons rich stream to produce a first PX-rich stream and afirst raffinate stream; (c) isomerizing at least a portion of the firstraffinate stream to produce a first isomerized stream having a higher PXconcentration than the first raffinate stream; (d) separating a secondportion of the C₈ hydrocarbons rich stream to produce a second PX-richstream and a second raffinate stream; (e) isomerizing, at leastpartially in the liquid phase, at least a portion of the secondraffinate stream to produce a second isomerized stream having a higherPX concentration than the second raffinate stream, where the secondisomerized stream has a benzene concentration of less than 1,000 ppm anda C₉+ hydrocarbons concentration of less than 5,000 ppm; (f) recoveringat least a portion of at least one of the first and second PX-richstreams as PX-rich product; and (g) providing at least a portion of thefirst isomerized stream and at least a portion of the second isomerizedstream to the separating (a).

Paragraph B: The process of paragraph A, where the second isomerizedstream has a higher PX concentration than the second raffinate streamand a benzene concentration of less than 500 ppm and a C₉+ hydrocarbonsconcentration of less than 3,000 ppm.

Paragraph C: The process of paragraphs A-B, where the second isomerizedstream has a higher PX concentration than the second raffinate streamand a benzene concentration of less than 300 ppm and a C₉+ hydrocarbonsconcentration of less than 2,000 ppm.

Paragraph D: The process of paragraphs A-C, where said step ofisomerizing (e) takes place at a temperature at or below 300° C.

Paragraph E: The process of paragraphs A-D, where said step ofisomerizing (e) takes place at a temperature at or below 280° C.

Paragraph F: The process of paragraphs A-E, where the flow rate of thesecond raffinate stream during said step of isomerizing (e) is at least0.1 WHSV.

Paragraph G: The process of paragraphs A-F, where the flow rate of thesecond raffinate stream during said step of isomerizing (e) is at least2.0 WHSV.

Paragraph H: The method of paragraphs A-G, where said step ofisomerizing (e) employs a molecular sieve catalyst.

Paragraph I: The method of paragraphs A-H, where said step ofisomerizing (e) employs a zeolite catalyst of MFI structure.

Paragraph J: The method of paragraphs A-I, where said step ofisomerizing (e) employs a ZSM-5 catalyst.

Paragraph K: The method of paragraphs A-J, where said step ofisomerizing (c) takes place at least partially in the vapor phase.

Paragraph L: The method of paragraphs A-K, where said step of recovering(f) includes selective adsorption.

Paragraph M: The method of paragraphs A-L, where said separating (d)includes fractional crystallization.

Paragraph N: The method of paragraphs A-M, providing at least a portionof the second raffinate stream to said separating (b).

Paragraph O: The method of paragraphs A-N, providing at least a portionof the second isomerized stream to said separating (b).

Paragraph P: A process for producing a PX-rich product, the processcomprising: (a) separating a feedstock containing C₈ hydrocarbons toproduce a C₈ hydrocarbons rich stream; (b) separating a first portion ofthe C₈ hydrocarbons rich stream to produce a first PX-rich stream and afirst raffinate stream; (c) isomerizing at least a portion of the firstraffinate stream to produce a first isomerized stream having a higher PXconcentration than the first raffinate stream; (d) separating the firstisomerized stream to produce a second C₈ hydrocarbons rich stream; (e)separating the second C₈ hydrocarbons rich stream to produce a secondPX-rich stream and a second raffinate stream; (f) isomerizing, at leastpartially in the liquid phase, at least a portion of the secondraffinate stream to produce a second isomerized stream having a higherPX concentration than the second raffinate stream; and (g) providing atleast a portion of the second isomerized stream to the separating (a).

Paragraph Q: The method of paragraph P, where said separating (b)includes selective adsorption.

Paragraph R: The method of paragraphs P-Q, where said step ofisomerizing (c) takes place at least partially in the vapor phase.

Paragraph S: The method of paragraphs P-R where said separating (e)includes fractional crystallization. Paragraph T: The process ofparagraphs

Paragraph T: The method of paragraphs P-S, where the second isomerizedstream has a benzene concentration of less than 1,000 ppm and a C₉+hydrocarbons concentration of less than 5,000 ppm.

Paragraph U: The method of paragraphs P-T, where said step ofisomerizing (f) takes place at a temperature at or below 300° C.

Paragraph V: The method of paragraphs P-U, providing at least a portionof the second raffinate stream to said separating (b).

Paragraph W: The method of paragraphs P-V providing at least a portionof the second isomerized stream to said separating (b).

Paragraph X: The method of paragraphs P-W, further comprising the stepof separating at least a portion of the second isomerized stream toremove at least a portion of C₇− and C₉+ hydrocarbons generated in saidisomerizing (f) and subsequently providing the second isomerized streamto said separating (b).

Paragraph Y: A process for producing a PX-rich product, the processcomprising: (a) separating a feedstock containing C₈ hydrocarbons toproduce a C₈ hydrocarbons rich stream; (b) separating a first portion ofthe C₈ hydrocarbons rich stream to produce a first PX-rich stream and afirst raffinate stream; (c) isomerizing at least a portion of the firstraffinate stream to produce a first isomerized stream having a higher PXconcentration than the first raffinate stream; (d) separating the firstraffinate stream to produce a second C₈ hydrocarbons rich stream; (e)separating the second C₈ hydrocarbons rich stream to produce a secondPX-rich stream and a third stream; (f) isomerizing, at least partiallyin the liquid phase, at least a portion of the second raffinate streamto produce a second isomerized stream having a higher PX concentrationthan the second raffinate stream, where the second isomerized stream hasa benzene concentration of less than 1,000 ppm and a C₉+ hydrocarbonsconcentration of less than 5,000 ppm; and (g) providing at least aportion of the second isomerized stream to the separating (a).

Paragraph Z: The process of paragraph Y, where the second isomerizedstream has a higher PX concentration than the second raffinate streamand a benzene concentration of less than 500 ppm, and a C₉+ hydrocarbonsconcentration of less than 3,000 ppm.

Paragraph AA: The process of paragraphs Y-Z, where the second isomerizedstream has a higher PX concentration than the second raffinate streamand a benzene concentration of less than 300 ppm, and a C₉+ hydrocarbonsconcentration of less than 2,000 ppm.

Paragraph BB: The process of paragraphs Y-AA, where said step ofisomerizing (f) takes place at a temperature at or below 300° C.

Paragraph CC: The process of paragraphs Y-BB, where said step ofisomerizing (f) takes place at a temperature at or below 280° C.

Paragraph DD: The process of paragraphs Y-CC, where the flow rate of thesecond raffinate stream during said step of isomerizing (f) is at least0.1 WHSV.

Paragraph EE: The process of paragraphs Y-DD, where the flow rate of thesecond raffinate stream during said step of isomerizing (f) is at least2.0 WHSV.

Paragraph FF: The method of paragraphs Y-EE, where said step ofisomerizing (f) employs a molecular sieve catalyst.

Paragraph GG: The method of paragraphs Y-FF, where said step ofisomerizing (f) employs a zeolite catalyst of MFI structure.

Paragraph HH: The method of paragraphs Y-GG, where said step ofisomerizing (f) employs a ZSM-5 catalyst.

Paragraph II: The method of paragraphs Y-HH, providing at least aportion of the second raffinate stream to said separating (b).

Paragraph JJ: The method of paragraphs Y-II, providing at least aportion of the second isomerized stream to said separating (b).

Paragraph KK: In a PX-production process where a C₈ hydrocarbon richstream is separated into a PX-rich stream and a PX-depleted stream byfractional crystallization, and where the PX-depleted stream issubsequently isomerized to increase the concentration of PX in thePX-depleted stream for subsequent separation by selective adsorption,the improvement comprising isomerizing said PX-depleted stream in theliquid phase to produce a stream having a higher PX concentration thanthe PX-depleted stream, a benzene concentration of less than 500 ppm,and a C₉+ concentration of less than 5,000 ppm.

Paragraph LL: The process of paragraph KK, where said stream having ahigher PX concentration than the PX-depleted stream has a benzeneconcentration of less than 500 ppm and a C₉+ hydrocarbons concentrationof less than 3,000 ppm.

Paragraph MM: The process of paragraphs KK-LL, where said stream havinga higher PX concentration than the PX-depleted stream has a benzeneconcentration of less than 300 ppm and a C₉+ hydrocarbons concentrationof less than 2,000 ppm.

Paragraph NN: The process of paragraphs KK-MM, where said step ofisomerizing the PX-depleted stream in the liquid phase takes place at atemperature at or below 300° C.

Paragraph OO: The process of paragraphs KK-NN, where said step ofisomerizing the PX-depleted stream in the liquid phase takes place at atemperature at or below 280° C.

Paragraph PP: The process of paragraphs KK-OO, where said step ofisomerizing the PX-depleted stream in the liquid phase takes place at aflow rate of at least 0.1 WHSV.

Paragraph QQ: The process of paragraphs KK-PP, where said step ofisomerizing the PX-depleted stream in the liquid phase takes place at aflow rate of at least 2.0 WHSV.

Paragraph RR: The method of paragraphs KK-QQ, where said step ofisomerizing the PX-depleted stream in the liquid phase employs amolecular sieve catalyst.

Paragraph SS: The method of paragraphs KK-RR, where said step ofisomerizing the PX-depleted stream in the liquid phase employs a zeolitecatalyst of MFI structure.

Paragraph TT: The method of paragraphs KK-SS, where said step ofisomerizing the PX-depleted stream in the liquid phase employs a ZSM-5catalyst.

Paragraph UU: A process for producing a PX-rich product, the processcomprising: (a) providing a PX-depleted stream; (b) isomerizing at leasta portion of the PX-depleted stream to produce an isomerized streamhaving a PX concentration greater than the PX-depleted stream and abenzene concentration of less than 1,000 ppm and a C₉+ hydrocarbonsconcentration of less than 5,000 ppm; and (c) separating the isomerizedstream by selective adsorption.

EXAMPLES

A C₈+ hydrocarbon stream was treated in accordance with the processdepicted in FIG. 2. Liquid-phase isomerization was conducted at 417using a ZSM-5 zeolite catalyst at 265 psig and 3 WHSV. The temperatureof isomerization was varied as set forth in Table II. The concentrationof various hydrocarbon constituents within the stream was measuredbefore and after the liquid-phase isomerization unit. Also, consistentwith FIG. 2, the stream was subsequently treated by vapor-phaseisomerization. The concentration of various constituents within thestream was measured before and after the vapor-phase isomerization.

TABLE II Examples — 1 Δ1 2 Δ2 3 Δ3 — 4 Δ4 Stream Feed to Liquid LiquidLiquid Liquid Liquid Liquid Feed to Vapor Vapor Liquid Phase Phase PhasePhase Phase Phase Vapor Phase Phase Phase effluent Effluent EffluentEffluent Effluent Effluent Phase effluent effluent Temp. — 246° C. 246°C. 241° C. 241° C. 236° C. 236° C. — — — Benzene 0.008 0.022 0.014 0.0190.011 0.015 0.007 0.001 4.95 4.949 Toluene 0.963 1.077 0.114 1.035 0.0721.006 0.043 0.485 1.595 1.11 Ethyl Benzene 2.956 2.881 −0.075 2.901−0.055 2.93 −0.026 9.281 1.856 −7.425 Para-Xylene 12.514 21.168 8.65420.184 7.67 19.035 6.521 0.971 20.345 19.374 Meta-Xylene 62.621 51.572−11.049 53.27 −9.351 54.907 −7.714 63.021 45.932 −17.089 Ortho-Xylene16.625 18.849 2.224 18.19 1.565 17.731 1.106 23.258 19.956 −3.302 C₉Aromatics 0 0.115 0.115 0.057 0.057 0.045 0.045 0.103 0.667 (as 0.564 C₉and C₁₀+) C₁₀+ Aromatics 0 0.082 0.082 0.041 0.041 0.033 0.033 — — —Non-Aromatics 4.31 4.231 −0.079 4.3 −0.01 4.295 −0.015 2.88 4.683 1.803Hydrogen — 0 0 0 0 0 0 0 0.016 0.016 Totals 99.997 99.997 0 99.997 099.997 0 100 99.333 —

The data in Table II shows that the production of benzene and toluene ismarkedly lower when liquid-phase isomerization is compared tovapor-phase isomerization. Also, the data shows the significanttemperature dependence of benzene production even in the liquid phase.Notably, at 246° C., the benzene concentration was at 220 ppm, which isbelow the 300 ppm threshold. While within critical thresholds, it isnoted that this amount represents nearly a two-fold increase in theamount of benzene present after liquid-phase isomerization at 236° C.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

What is claimed is:
 1. In a PX-production process where a C₈ hydrocarbonrich stream is separated into a PX-rich stream and a PX-depleted streamby fractional crystallization, and where the PX-depleted stream issubsequently isomerized to increase the concentration of PX in thePX-depleted stream for subsequent separation by selective adsorption,the improvement comprising isomerizing said PX-depleted stream in theliquid phase to produce a stream having a higher PX concentration thanthe PX-depleted stream, a benzene concentration of less than 300 ppm,and a C₉+ hydrocarbon concentration of less than 3,000 ppm.
 2. Theprocess of claim 1, wherein said selective adsorption produces a PX-richstream and a PX-depleted stream.
 3. The process of claim 1, wherein atleast a portion of said isomerized stream is treated to remove said C9+hydrocarbon prior to said selective adsorption.
 4. The process of claim1, wherein at least a portion of said isomerized stream is treated toremove said benzene prior to said selective adsorption.